1. Field of the Invention
The present invention relates to a channel assignment system for use with TDMA (Time Division Multiple Access) communication system for transmitting information from a base station to a terminal unit corresponding to a transmission request thereof with a plurality of channels, in particular, to a channel assignment system of which the base station assigns channels with small interference as communication channels with the terminal unit corresponding to history of the used channels so as to improve the quality of uplink channels used to transmit data from the terminal unit to the base station.
2. Description of the Related Art
The channel assignment system for assigning channels with small interference corresponding to history thereof has been disclosed in for example Japanese Patent Laid-Open Publication 61-244137 as a related art reference. This related art reference is a radio communication system having a function for starting communication after verifying that a channel to be used is a blank channel. Individual channels are assigned priority levels. Channels are preferentially used in the order of higher priority levels by turns. The priority levels of the channels are variably assigned corresponding to the history that have been used in the channel.
FIG. 6 is a block diagram showing the structure of the conventional channel assignment system. As shown in FIG. 6, this system comprises a mobile station terminal (MS) 610, an antenna 621, a modulator (MOD) 622, a synthesizer (SYN) 623, a microprocessor (MPU) 624, a demodulator (DEM) 625, and a power detector (DET) 626.
In this system, at a communication request wait state, the microprocessor 624 selects a control channel as a communication channel and controls the synthesizer 623. A communication request of the mobile station terminal 610 is sent through the control channel. When the mobile station terminal 610 issues a communication request, a signal of the communication request is input from the antenna 621. The demodulator 625 demodulates the signal and inputs the demodulated signal to the microprocessor 624. The microprocessor 624 starts the channel selecting operation. The power detector 626 measures the power of the received signal of the channel selected by the microprocessor 624 and supplies to the microprocessor 624 a determination signal that represents whether or not the power of the received signal exceeds a predetermined threshold value.
FIG. 7 is a flow chart showing the channel selecting operation of the microprocessor 624.
When there is no communication request, the microprocessor 624 waits until a communication request is received through the control channel (at step 701). When the microprocessor 624 receives a communication request (at step 702), the microprocessor 624 selects a channel with the highest priority level (at steps 703 and 704).
When this channel is busy, the microprocessor 624 selects a channel with the next highest priority level. After repeating this operation, if a channel with the lowest priority level is busy, the microprocessor 624 sends a call loss signal to the mobile station terminal 610 (at steps 704 and 707 to 709).
Even if the selected channel is not busy (namely, the determined result at step 704 is No), when the determination signal that the power of the received signal exceeds a predetermined threshold value, has been supplied from the power detector 626 (at step 705), since the power of the interference wave is large, the microprocessor 624 determines that this channel cannot be used, decreases the priority level of the channel (at step 706), and selects a channel with the next highest priority level (at steps 707 and 708).
After repeating this operation, when the microprocessor 624 had determined that a channel with the lowest priority level cannot be used, it sends a call loss signal to the mobile station terminal 610 (at step 709).
When the selected channel is not busy and the microprocessor 624 has received the determination signal that the power of the received signal does not exceed a predetermined threshold value (namely, the determined result at step 705 is No), the microprocessor 624 increases the priority level of the channel (at step 710), and assigns the channel with the increased highest priority level (at step 711).
When such an operation is performed corresponding to each communication request, the priority level of a channel with small interference increases, whereas the priority level of a channel with large interference decreases. In other words, the microprocessor 624 can learn the interference distribution of each channel. When channels are selected in the order of higher priority levels, channels with small interference can be used. Thus, the channel quality of the entire system can be improved.
In the conventional channel controlling system shown in FIG. 6, since each mobile station terminal uses one whole channel, when a channel is selected, the amount of interference can be measured corresponding to the power of the received signal of the channel.
However, in the TDMA system, since one channel is divided into time slots, one channel is shared by a plurality of mobile station terminals. Thus, when the amount of interference is measured by the conventional channel controlling system, the power of the received signal should be measured at a time of a blank slot. Thus, a complicated control process is required.